In recent years, in ultrahigh-speed communication beyond 100 G bit/second, a communication technology by DP-QPSK (Dual Polarization Differential Quadrature Phase Shift Keying) excelling in wavelength utilization efficiency, receiving characteristics and dispersion compensation ability has been attracting attention. A receiver in a DP-QPSK method needs to have a function to separate a light signal into a TE (Transverse Electric) and a TM (Transverse Magnetic) polarization components, and a 90 degree optical hybrid function for retrieving phase information from these polarized light signals. This phase information consists of four values on an I-Q plane including Ip and In whose phases differ by π, and Qp and Qn having a phase delay of π/2 to Ip and In, respectively.
Because a planar lightwave circuit using an optical waveguide technology is dominant as a device which realizes the function of a receiver of such DP-QPSK method, development thereof has been advanced in recent years. An optical waveguide technology is a technology for forming an optical waveguide of various shapes on a substrate using the same micro fabrication technologies as semiconductor integrated circuit manufacturing process, and it is suited to integration and mass production.
For example, in a related art document (Toshikazu Hashimoto, et al., “Dual polarization optical hybrid module using planar lightwave circuit”, Proceedings of the 2009 IEICE Electronics Society Conference 1, Sep. 15, 2009, p. 194), a lightwave circuit structure shown in FIG. 6 is disclosed. This lightwave circuit has a general lightwave circuit structure in which the polarization splitting function and the 90 degree optical hybrid function which have been mentioned above are integrated on a planar optical circuit. FIG. 7 indicates the structure of the planar optical circuit for the TE light signal in the 90 degree optical hybrid as a schematic diagram.
In FIG. 7, optical branch devices 16 and 17, optical waveguide arms 18-21, and optical couplers 22 and 23 having two inputs and two outputs are indicated. Arms 18-21 constitute an interferometer. Lengths for the arms 18-20 are the same. Length of the arm 21 is longer than those of the other arms by one-fourth of the transmitted light wavelength traveling along the optical waveguide, so that a phase of light traveling along the arm 21 is delayed. Using this configuration, four values of phase information on the I-Q plane are outputted from optical couplers 22 and 23, and thus the above-mentioned 90 degree optical hybrid function is realized.